1. Field of the Invention
This invention relates to a semiconductor integrated circuit device and more particularly to a magnetic semiconductor memory having magneto-resistive effect elements.
2. Description of the Related Art
As one of nonvolatile semiconductor memories of a next generation, much attention is given to a magnetic semiconductor memory. Each of memory cells in the semiconductor memory includes a magneto-resistive effect element whose resistance varies according to the magnetization direction thereof and utilizes the magneto-resistive effect element as information storage means. The magneto-resistive effect element includes a fixed layer (pinned layer) in which the magnetization direction is fixed and a free layer in which the magnetization direction can be freely changed. The resistance thereof is changed according to whether the magnetization direction of the free layer is parallel or anti-parallel to the magnetization direction of the fixed layer. A variation in the resistance is set to correspond to “0”, “1” of storage information, for example.
Further, in the recent magnetic semiconductor memory, a tunneling magneto-resistive element which has a tunnel insulating film between the free layer and the fixed layer and in which a difference (MR ratio) between the resistance at the parallel time and the resistance at the anti-parallel time is set large so that whether storage information is “0” or “1” can be easily determined is developed. The magnetic semiconductor memory is described in a known document, for example, document 1.
Document 1: Roy Scheuerlein et al., “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”, 2000 IEEE International Solid-State Circuits Conference, Feb. 8, 2000, p128–129.
The magnetic semiconductor memory has received much attention as one of the nonvolatile semiconductor memories of the next generation, but it has some problems to be solved for practical use.
One of the problems is that a write current used to write information into the magneto-resistive effect element is large. There are various reasons why the write current becomes large and one of the reasons as viewed from the practical viewpoint is that a magnetic field (which is hereinafter referred to as a switching magnetic field) required for changing the magnetization direction of the free layer must have certain strength. If the switching magnetic field is weak, for example, a variation in the switching magnetic field caused by a thermal disturbance cannot be absorbed as a margin and there occurs a possibility that storage information will be unexpectedly destroyed. Under this circumstance, the switching magnetic field is required to have certain strength.
In order to generate a strong magnetic field, a large write current is required. If a large write current is used, a transistor having large driving ability becomes necessary. In order to design a transistor having large driving ability, sufficiently large channel width is required. This increases an area of the peripheral circuit of the magnetic semiconductor memory and makes it difficult to meet the market requirement of “large-scale memory capacity and small chip”.
Further, in order to cause a large current flow, a heavy load tends to be applied to the wirings of the semiconductor integrated circuit device and a bad influence will be given to the reliability of the wirings, for example, the reliability relating to the service life of the wirings.
In order to prevent an increase in the area of the chip and solve the problem of deterioration of the reliability of the wirings caused by passing a large current, it is preferable to enhance the efficiency of generation of a magnetic field with respect to a current and generate a stronger magnetic field by use of a smaller current. Simply stated, the wrings are surrounded by a magnetic body and the magnetic body is magnetized by a magnetic field generated by a current. This is a yoke. The wirings having a yoke can generate a stronger magnetic field by a small current in comparison with the wirings having no yoke. Thus, the write current can be reduced.
However, in order to form the wirings having the yoke, it is necessary to surround the wirings with the magnetic body. Therefore, the process becomes complicated and more difficult. Of course, the number of manufacturing steps of forming the magnetic body increases. Further, factors to be made clear, for example, the influence given to the wirings by the yoke, the influence by the wirings having the yoke on an integrated circuit and the like must be considered.